In-Flight Alloying of Nanocrystalline Yttria-Stabilized Zirconia Using Suspension Spray to Produce Ultra-Low Thermal Conductivity Thermal Barriers

摘要

Previous researchers have shown that it is possible to combine rare-earth oxides with the standard thermal barrier coating material (4.5 mol% Y_2O_3-ZrO_2 or YSZ) to form ultra-low thermal conductivity coatings using a standard powder manufacturing route. A similar approach to making low thermal conductivity coatings by adding rare-earth oxides is discussed presently, but a different manufacturing route was used. This route involved dissolving hydrated ytterbium and neodymium nitrates into a suspension of 80 nm diameter 4.5 mol% YSZ powder and ethanol. Suspension plasma spray was then used to create coatings in which the YSZ powders were alloyed with rare-earth elements while the plasma transported the melted powders to the substrate. Mass spectrometry measurements showed a YSZ coating composition, in mol%, of ZrO_2-4.4 Y_2O_3-1.4 Nd_2O_3-1.3 Yb_2O_3. The amount of Yb~(3+) and Nd~(3+) ions in the final coating was ～50% of that added to the starting suspension. Wide-angle X-ray diffraction revealed a cubic ZrO_2 phase, consistent with the incorporation of more stabilizer into the zirconia crystal structure. The total porosity in the coatings was ～35-36%, with a bulk density of 3.94 g/cm~3. Small-angle X-ray scattering measured an apparent void specific surface area of ～2.68 m /cm for the alloyed coating and ～3.19 m~2 /cm~3 for the baseline coating. Thermal conductivity (k_(th)) of the alloyed coating was ～0.8W/m/K at 800℃, as compared with ～1.5 W/m/K at 800℃ for the YSZ-only baseline coating. After 50 h at 1200℃, k_(th) increased to ～1.1 W/m/K at 800℃ for the alloyed samples, with an associated decrease in the apparent void specific surface area to ～1.55 m~2/cm~3.